Klemow, Kenneth M. and Malika Mohseni. 1996. HOW ACCURATE ARE THE NATIONAL WETLAND INVENTORY MAPS: AN ANALYSIS FROM NORTHEASTERN PENNSYLVANIA. Contributed paper: Ecological Society of America meeting. Abstract: Bulletin of the Ecological Society of America 77:237.
Individuals seeking to determine whether an area is a wetland often consult the National Wetland Inventory (NWI) maps produced by the US Fish & Wildlife Service. Despite their popularity, wetland scientists often perceive NWI maps as being misleading because many areas mapped as upland prove to be wetland when examined in the field. Alternatively, wetland scientists often rely on county Soil Surveys that are more detailed and classify soils according to their drainage class. To determine the degree of correlation between NWI maps and soil surveys, an analysis was conducted for a 30 sq. mi. area in northeastern Pennsylvania. Soils maps were digitized and mapping units were assigned to five categories based on drainage class. The locations of the NWI wetlands were superimposed on the soils maps, and the proportion of each soil class that was in and out of mapped wetland was determined. The analysis found that 59% of all soils mapped as poorly drained was not designated as wetland by NWI. Further, 96% of soils mapped as somewhat poorly drained (and frequently wetland) was designated as upland by NWI. This analysis reveals that NWI maps can seriously underestimate the extent of wetland-prone areas.